The device may more exactly be intended for connecting and disconnecting objects in electric power plants or electric power net works as well as connecting and disconnecting parts thereof to or from other equipment included in the electric power plant or an object connected thereto. Accordingly, the term "object" is intended to have a very broad meaning and comprises any apparatuses and devices included in electric power plants and electric power networks as well as generally parts of the electric power plant and/or the electric power network.
It may as an example be mentioned that the object may be an electric apparatus having a magnetic circuit, for example a generator, transformer or motor Also other objects are conceivable, for instance power lines and cables, switch-gear equipment etc. The present invention is intended to be used for medium and high voltages. According to the IEC-standard medium voltage means 1-72,5 kV, while high voltage is &gt;72,5 kV. Accordingly, the transmission, subtransmission and distribution levels are included.
In electric power plants known circuit breakers, for instance SF.sub.6 -breakers, oil breakers or so-called vacuum breakers, have normally been used for connection and disconnection of the object in question. In some rare cases, in which there is a requirement of a very high speed, semiconductor "breakers", such as for example thyristors or IGBTs, may be used.
All such circuit breakers are so designed that they when breaking give rise to a galvanic separation of two metal contacts (arcing contacts), between which the current to be interrupted continues to flow in an arc. The interruption or breaking is then achieved by arranging the breaker so that this arc is extinguished upon a current zero passage, i.e. when the current through the breaker arrives at zero and change polarity, which takes place two times each twenty milliseconds in a 50 Hz-network. Accordingly, these circuit breakers function only for alternating current and not for direct current, where no zero passage occurs.
A circuit breaker with the construction according to above has to be designed to interrupt both in a large amount of breaking cases with comparatively moderate currents, so-called operation currents, but also in breaking cases with a high overcurrent, fault currents.
A circuit breaker has to be designed to be able to handle large amounts of energy when breaking an overcurrent in the arc between the arcing contacts. The gap between the contacts has to be brought to a very high dielectric strength within a short period of time after a current breaking has been successfully carried out so as to avoid reignition of an arc, i.e. guarantee the continued existence of the interruption.
Since circuit breakers, for example SF.sub.6 -breakers, oil breakers or so-called vacuum breakers, have to handle a high thermal and electric load in one and the same critical region within a short period of time, they will get a comparatively complex construction, which results in a comparatively long breaking time.
It is underlined that the overcurrent primarily intended here is a short-circuit current generated in connection to the object switched, for example as a consequence of a fault in the electric insulation system of the object switched. Such faults mean that the fault current (short-circuit current) of external network/equipment will tend to flow through an arc. This may lead to failure. It may also be mentioned that the maximum short-circuit current (fault current) the Swedish power network is dimensioned for is 63 kA. The short-circuit current may in reality be 40-50 kA.
A problem with such circuit breakers is the long breaking time thereof. The maximum breaking time (IEC-standard) for a breaking completely carried out is 150 milliseconds (ms). It is difficult to reduce this breaking time to under 90-130 ms depending on the operation case. The consequence of this is that a very high current will flow through the object switched upon a fault therein during the entire time required for bringing the circuit breaker to interruption. The total fault current of the external power network means a considerable stress on the object switched during this time. The operation of the network will during this time also be disturbed, so that other equipment connected to the network may be substantially disturbed or damaged. In order to avoid damages and total breakdowns with respect to the object switched this is constructed so that it may manage to be excerted to the short-circuit currents/fault currents during the breaking time of the circuit breaker without any damages worth mentioning. The need to construct the object switched so that it may take the short-circuit current/fault current during a considerable time results in substantial drawbacks in the form of more expensive constructions and lower performances. With respect to disturbances of the network and equipment connected thereto there is presently no protection integrated in the network, so that each manufacturer has to protect sensitive equipment with "backup" and network stabilizing assemblies. More sensitive equipment such as systems based on microprocessors, for example communication and computer systems, frequently requires a restart associated with considerable costs.
Semiconductor power devices, such as thyristors, MOSFETs and IGBTs, may not alone withstand the voltages in question, so that a number thereof have to be connected in series. Hundreds of such components have to be connected in series in some high voltage applications. This leads to a complicated control system of the equipment for ensuring the operation, i.e. that the voltage and power is distributed uniformly over the components. The use of semiconductor components made of silicon also results in comparatively high losses, which in turn, requires an efficient cooling, since the component may otherwise break down thermally. The total system with control, regulation and cooling all of the components connected in series individually on the individual voltage level thereof tends to become very complex and the entire system is therefore very costly. The costs may exceed those for circuit breakers considerably, which generally excludes the use of such semiconductor components in electric power plants and electric power networks for the applications discussed here.